The present invention relates to a piston for a hydraulic pressure chamber, especially a piston bore of a hydraulic brake. The Piston is configured as a bowl-shaped hollow cylinder that is open on one side and made from a metal blank of a constant wall thickness in a forming operation.
DE40 03 731 A1 discloses a cold-worked piston for a hydraulically operating brake which is preferably made in a deepdrawing process, and which is comparatively thin-walled and hence, has a lighter design compared to the cast pistons known from the state of the art. In order to protect the walls of the piston against deformation when subjected to force, the prior art piston has a curved bottom and a groove that is rolled up at its circumference, both reinforcing the shape of the piston. Due to the thin-walled design of steel pistons of this type compared to pistons manufactured from gray cast iron, it is achieved that also the frontal abutment surface of the bowl rim at the open piston end in application against the brake lining is considerably smaller compared to cast pistons. The result is a high surface pressure, which is not desirable. In addition, it has shown that a piston of this type, especially with a thin-walled configuration, suffers from inadmissibly great deformations under stress.
Based on the known state of the art, an object of the present invention is to provide a piston which permits being manufactured by means of a simple forming operation, which can be configured as light as possible and with low expenditure in material, without limiting the stress capacity of the piston. This object is achieved by a piston embodying the features of claim 1. The principle of the present invention involves making the piston from a metal blank which is as thin-walled as possible and has a constant wall thickness, especially in the shape of a ronde, by a non-cutting forming process, especially a cold forming process, in order to reduce weight compared to cast pistons known in the art. To increase the stress capacity of the piston, the piston is reinforced at the endangered portions by taking forming measures at the metal blank. In a very favorable manner, the piston may include folded portions in the areas which are subjected to particularly high stress, with the result that the thickness of the piston wall or of the piston bottom is increased compared to the thickness of the remaining piston wall or of the bottom. All in all, the thickness in these reinforced portions corresponds to several times the wall thickness of the metal blank. Compared to prior art piston designs, a considerable weight reduction is thereby achieved, and the necessary piston strength and piston rigidity is maintained.
In the capacity of areas which are highly stressed when subjected to force, above all the rim at the open end of the bowl-shaped piston and the transition area between the piston wall and the piston bottom are furnished with circumferential fold shapings that extend about the piston axis to reinforce these portions. Shaping a fold at the rim along with preferably doubling the wall thickness of the metal blank additionally achieves an increase in the circular abutment surface provided at the rim.
It is especially advantageous that the new piston can be employed in a hydraulically operating brake. The bottom of brake pistons of this type is usually exposed to the hydraulic pressure, and the frontal abutment surface at the open piston end bears against a brake lining. After the non-cutting forming process of the ronde-shaped metal blank, the piston rim at its end surface preferably undergoes a machining operation in order to enhance the accuracy of measurement of the smooth abutment surface. Due to the fold shaping, the abutment surface is increased compared to the remaining cross-sectional surface of the thin piston wall and, thus, reduces the surface pressure on the brake lining caused by the piston.
In a preferred aspect of the present invention, the piston wall of the piston has a radial step which, along with the folded portion at the piston rim, forms a circumferential groove about the piston axis on the outward peripheral surface of the piston. When the piston is installed into the hydraulic brake, a groove of this type may be used in particular to receive a ring seal or any other sealing element. This eliminates the need for a metal cutting process on the piston to provide the groove as done with previously known cast pistons. In addition, the circumferential radial step on the inner surface of the piston wall is used as a supporting surface for a lining holding spring which extends into the piston interior and may backgrip the step.
In another favorable embodiment of the piston, the piston bottom has a curved design, with the outward surface of the bottom exhibiting a convex curvature. This permits designing the piston with a very small thickness of the wall, as the stress capacity requirement may be, and hence with a minimum expenditure in material, while encountering only minor elastic deformations under stress.